ZIP-0701: ZRC-721 Non-Fungible Token Standard

Abstract

This proposal defines ZRC-721, the non-fungible token standard for the Zoo L2 chain. ZRC-721 extends the Lux LRC-721 standard (LP-3721) and is fully ERC-721 compatible. It adds three conservation-specific features: structured species metadata following IUCN taxonomy, an immutable provenance chain recording the lifecycle of wildlife-related assets, and a royalty mechanism (EIP-2981) where secondary sale royalties are directed to verified conservation funds. ZRC-721 is the foundation for wildlife adoption certificates, habitat deeds, and research data NFTs across the Zoo ecosystem.

Motivation

Wildlife conservation requires verifiable digital ownership primitives that:

Carry Scientific Metadata: Each NFT representing a species, habitat, or research output must embed standardized taxonomic and geospatial data.
Maintain Provenance: The full history of an asset -- minting, ownership, conservation actions, field observations -- must be on-chain and auditable.
Fund Conservation: Secondary market activity must direct royalties to conservation, not just creators.
Bridge Across Chains: Wildlife NFTs must be portable between Zoo L2 and Lux C-Chain per LP-3800.
Support Token-Bound Accounts: Each NFT must be compatible with ZIP-0703 (TBA) so wildlife NFTs can own assets.

Existing ERC-721 standards lack structured metadata schemas for ecological data and have no mechanism for directing royalties to mission-aligned causes.

Specification

Core Interface

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.20;

import {IERC721} from "@openzeppelin/contracts/token/ERC721/IERC721.sol";
import {IERC721Metadata} from "@openzeppelin/contracts/token/ERC721/extensions/IERC721Metadata.sol";
import {IERC2981} from "@openzeppelin/contracts/interfaces/IERC2981.sol";

/**
 * @title IZRC721
 * @notice Zoo ecosystem NFT standard with species metadata, provenance, and conservation royalties.
 * @dev Extends ERC-721 + ERC-2981 + LRC-721 (LP-3721).
 */
interface IZRC721 is IERC721, IERC721Metadata, IERC2981 {

    // ──────────────────────────────────────────────
    // Species Metadata
    // ──────────────────────────────────────────────

    struct SpeciesMetadata {
        string scientificName;      // e.g. "Panthera tigris"
        string commonName;          // e.g. "Bengal Tiger"
        string iucnStatus;          // CR, EN, VU, NT, LC, DD, NE
        string taxonomyClass;       // Mammalia, Aves, Reptilia, etc.
        string taxonomyOrder;       // Carnivora, Passeriformes, etc.
        string taxonomyFamily;      // Felidae, Accipitridae, etc.
        int64 latitude;             // Fixed-point (6 decimals) * 1e6
        int64 longitude;            // Fixed-point (6 decimals) * 1e6
        string habitatType;         // Tropical forest, Coral reef, etc.
        string region;              // Geographic region / country
        uint256 populationEstimate; // Best current estimate (0 = unknown)
    }

    /**
     * @notice Returns the species metadata for a given token.
     */
    function speciesMetadata(uint256 tokenId) external view returns (SpeciesMetadata memory);

    /**
     * @notice Sets species metadata. Only callable by token minter or authorized role.
     */
    function setSpeciesMetadata(uint256 tokenId, SpeciesMetadata calldata metadata) external;

    // ──────────────────────────────────────────────
    // Provenance Chain
    // ──────────────────────────────────────────────

    struct ProvenanceRecord {
        uint256 timestamp;
        address recorder;
        string action;         // "minted", "observed", "transferred", "conserved", "verified"
        string details;        // IPFS CID or UTF-8 description
        bytes32 evidenceHash;  // keccak256 of supporting evidence
    }

    /**
     * @notice Returns the full provenance chain for a token.
     */
    function provenanceChain(uint256 tokenId) external view returns (ProvenanceRecord[] memory);

    /**
     * @notice Returns the number of provenance records for a token.
     */
    function provenanceCount(uint256 tokenId) external view returns (uint256);

    /**
     * @notice Appends a provenance record. Callable by token owner or authorized verifiers.
     */
    function addProvenanceRecord(
        uint256 tokenId,
        string calldata action,
        string calldata details,
        bytes32 evidenceHash
    ) external;

    event ProvenanceRecorded(
        uint256 indexed tokenId,
        address indexed recorder,
        string action,
        uint256 timestamp
    );

    // ──────────────────────────────────────────────
    // Conservation Royalties
    // ──────────────────────────────────────────────

    /**
     * @notice Returns the conservation fund receiving royalties for a token.
     */
    function conservationFund(uint256 tokenId) external view returns (address);

    /**
     * @notice Returns the royalty split: (conservationBps, creatorBps).
     *         Total of both MUST equal the ERC-2981 royalty fraction.
     */
    function royaltySplit(uint256 tokenId) external view returns (uint16 conservationBps, uint16 creatorBps);

    /**
     * @notice Sets the conservation fund and royalty split for a token.
     *         Conservation share MUST be >= 50% of total royalty.
     */
    function setConservationRoyalty(
        uint256 tokenId,
        address fund,
        uint16 conservationBps,
        uint16 creatorBps
    ) external;

    event ConservationRoyaltySet(
        uint256 indexed tokenId,
        address indexed fund,
        uint16 conservationBps,
        uint16 creatorBps
    );
}

Metadata Schema (JSON)

ZRC-721 tokens MUST return a tokenURI that resolves to JSON conforming to the following schema. This extends ERC-721 Metadata JSON Schema with Zoo-specific fields.

{
  "name": "Bengal Tiger #42",
  "description": "Adoption certificate for a tracked Bengal tiger in Sundarbans.",
  "image": "ipfs://Qm.../tiger-42.jpg",
  "external_url": "https://zoo.ngo/wildlife/42",
  "attributes": [
    { "trait_type": "Scientific Name", "value": "Panthera tigris" },
    { "trait_type": "IUCN Status", "value": "EN" },
    { "trait_type": "Taxonomy Class", "value": "Mammalia" },
    { "trait_type": "Habitat", "value": "Mangrove Forest" },
    { "trait_type": "Region", "value": "Bangladesh" },
    { "trait_type": "Population Estimate", "value": 106 },
    { "display_type": "number", "trait_type": "Conservation Credits Generated", "value": 2500 }
  ],
  "zoo": {
    "speciesId": "panthera-tigris",
    "provenanceCID": "ipfs://Qm.../provenance.json",
    "conservationFund": "0x1234...abcd",
    "latitude": 21.9497,
    "longitude": 89.1833,
    "dataSource": "Wildlife Conservation Society",
    "lastObservation": "2025-01-10T08:30:00Z"
  }
}

Provenance Chain Rules

The first provenance record is automatically created at mint time with action "minted".
Provenance records are append-only. No record may be deleted or modified.
Authorized verifiers (registered in ZIP-0100 Contract Registry) may add records with action "verified".
The token owner may add records with actions "observed" or "conserved".
On transfer, a "transferred" provenance record is automatically appended.
evidenceHash MUST be keccak256(abi.encodePacked(evidenceCID)) where evidenceCID is the IPFS content identifier of supporting media.

Royalty Distribution

ZRC-721 implements ERC-2981 (royaltyInfo). The returned royalty is split between the conservation fund and the original creator:

Conservation share MUST be at least 50% of the total royalty.
Default total royalty is 7.5% (750 basis points): 5% conservation, 2.5% creator.
Marketplaces querying royaltyInfo receive the total amount and fund address. The contract internally splits and forwards via a RoyaltySplitter contract.

Bridgeable Extension

interface IZRC721Bridgeable is IZRC721 {
    function bridge() external view returns (address);
    function lockForBridge(uint256 tokenId, bytes32 destinationChainId) external;
    function mintFromBridge(
        address to, uint256 tokenId,
        SpeciesMetadata calldata metadata,
        bytes32 sourceChainTxHash
    ) external;
    event BridgeLock(uint256 indexed tokenId, bytes32 indexed destChainId);
    event BridgeMint(uint256 indexed tokenId, bytes32 indexed sourceTxHash);
}

When bridging, species metadata and the latest provenance snapshot are included in the bridge message. The full provenance chain is stored on IPFS and referenced by CID.

Rationale

Why structured on-chain metadata instead of just tokenURI? Off-chain metadata can disappear. On-chain species data ensures the conservation identity of an NFT survives even if IPFS gateways or centralized hosting fail.

Why append-only provenance? Immutability is essential for trust. Conservation donors need assurance that the history of a wildlife NFT cannot be retroactively altered.

Why require >= 50% royalty to conservation? The Zoo ecosystem's primary purpose is conservation (ZIP-0500). Requiring majority royalty allocation to conservation ensures NFT markets generate sustained funding regardless of creator incentives.

Why ERC-2981? It is the established royalty standard with broad marketplace support (OpenSea, Blur, LooksRare). Using a custom royalty mechanism would reduce adoption.

Backwards Compatibility

ZRC-721 is fully backwards compatible with ERC-721 (EIP-721) and ERC-2981 (EIP-2981). Standard NFT wallets, marketplaces, and indexers will function correctly. The species metadata, provenance, and conservation royalty interfaces use novel function selectors that do not conflict with existing standards. ERC-165 supportsInterface MUST return true for IZRC721, IERC721, IERC721Metadata, IERC2981, and IERC165.

Security Considerations

Provenance Spam: Rate-limiting provenance record additions prevents storage bloat attacks. Implementations SHOULD cap records per token or charge gas proportional to storage.
Verifier Authorization: Only addresses registered in the Zoo Contract Registry (ZIP-0100) with the VERIFIER_ROLE may add "verified" provenance records.
Royalty Enforcement: On-chain royalties are not enforceable at the protocol level on all marketplaces. Implementations SHOULD use operator filter registries where available.
Metadata Immutability: Once species metadata is set and a "verified" provenance record exists, metadata SHOULD be locked to prevent retroactive changes.
Bridge Replay: mintFromBridge MUST track processed sourceChainTxHash values to prevent duplicate minting.
Geolocation Privacy: Exact coordinates of endangered species could aid poachers. Implementations SHOULD truncate coordinates to 0.1-degree precision for publicly visible metadata and store precise coordinates in encrypted off-chain storage accessible only to authorized researchers.

ZRC-721: Non-Fungible Token Standard